Imaging pixels with plasmonic color filter elements

ABSTRACT

Image sensors may include plasmonic color filter elements that transmit specific wavelengths of incident light. Each plasmonic color filter element may be interposed between a respective microlens and photosensitive area. The plasmonic color filter elements may be formed from a metal layer such as gold, silver, platinum, aluminum, or copper and may have a pattern of openings in the metal layer that is designed to allow transmission of a certain type of light. To prevent cross-talk between adjacent pixels having plasmonic color filter elements, metal walls may be interposed between adjacent plasmonic color filter elements. The metal walls may extend above the upper surface of the metal layer that forms the plasmonic color filter elements. The metal walls may run around the periphery of each plasmonic color filter element.

BACKGROUND

This relates generally to imaging devices, and more particularly, toimaging devices having color filter elements.

Image sensors are commonly used in electronic devices such as cellulartelephones, cameras, and computers to capture images. In a typicalarrangement, an image sensor includes an array of image pixels arrangedin pixel rows and pixel columns. Circuitry may be coupled to each pixelcolumn for reading out image signals from the image pixels.

Conventional imaging pixels are covered by color filter elements formedfrom organic material. The color filter elements filter incident lightto allow only light of a desired wavelength to reach the underlyingpixel. However, the color filter elements of conventional imaging pixelsmay require different materials to transmit different colors of incidentlight and may have wider than desired transmission profiles.

It would therefore be desirable to be able to provide improvedarrangements for color filter elements in image sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative electronic device having an imagesensor in accordance with an embodiment.

FIG. 2 is a diagram of an illustrative pixel array and associatedreadout circuitry for reading out image signals in an image sensor inaccordance with an embodiment.

FIG. 3 is a cross-sectional side view of an illustrative image sensorhaving plasmonic color filter elements in accordance with an embodiment.

FIG. 4A is a top view of an illustrative image sensor having plasmoniccolor filter elements of the type shown in FIG. 3 in accordance with anembodiment.

FIG. 4B is a top view of an illustrative image sensor having plasmoniccolor filter elements and a reduced fill factor to reduce cross-talk inaccordance with an embodiment.

FIG. 4C is a top view of an illustrative image sensor having plasmoniccolor filter elements and metal walls to reduce cross-talk in accordancewith an embodiment.

FIG. 5A is a graph showing the response of pixels having the blue,green, and red color filter elements of the image sensor in FIG. 4A inaccordance with an embodiment.

FIG. 5B is a graph showing the response of pixels having the blue,green, and red color filter elements of the image sensor in FIG. 4B inaccordance with an embodiment.

FIG. 5C is a graph showing the response of pixels having the blue,green, and red color filter elements of the image sensor in FIG. 4C inaccordance with an embodiment.

FIG. 6 is a cross-sectional side view of an illustrative image sensorthat has plasmonic color filter elements and metal walls such as theimage sensor shown in FIG. 4C in accordance with an embodiment.

FIG. 7 is a top view of an illustrative image sensor having nineplasmonic color filter elements in a 3×3 unit square and metal walls inaccordance with an embodiment.

DETAILED DESCRIPTION

Embodiments of the present invention relate to image sensors. It will berecognized by one skilled in the art that the present exemplaryembodiments may be practiced without some or all of these specificdetails. In other instances, well-known operations have not beendescribed in detail in order not to unnecessarily obscure the presentembodiments.

Electronic devices such as digital cameras, computers, cellulartelephones, and other electronic devices may include image sensors thatgather incoming light to capture an image. The image sensors may includearrays of pixels. The pixels in the image sensors may includephotosensitive elements such as photodiodes that convert the incominglight into image signals. Image sensors may have any number of pixels(e.g., hundreds or thousands or more). A typical image sensor may, forexample, have hundreds of thousands or millions of pixels (e.g.,megapixels). Image sensors may include control circuitry such ascircuitry for operating the pixels and readout circuitry for reading outimage signals corresponding to the electric charge generated by thephotosensitive elements.

FIG. 1 is a diagram of an illustrative imaging and response systemincluding an imaging system that uses an image sensor to capture images.System 100 of FIG. 1 may an electronic device such as a camera, acellular telephone, a video camera, or other electronic device thatcaptures digital image data, may be a vehicle safety system (e.g., anactive braking system or other vehicle safety system), or may be asurveillance system.

As shown in FIG. 1, system 100 may include an imaging system such asimaging system 10 and host subsystems such as host subsystem 20. Imagingsystem 10 may include camera module 12. Camera module 12 may include oneor more image sensors 14 and one or more lenses.

Each image sensor in camera module 12 may be identical or there may bedifferent types of image sensors in a given image sensor arrayintegrated circuit. During image capture operations, each lens may focuslight onto an associated image sensor 14. Image sensor 14 may includephotosensitive elements (i.e., pixels) that convert the light intodigital data. Image sensors may have any number of pixels (e.g.,hundreds, thousands, millions, or more). A typical image sensor may, forexample, have millions of pixels (e.g., megapixels). As examples, imagesensor 14 may include bias circuitry (e.g., source follower loadcircuits), sample and hold circuitry, correlated double sampling (CDS)circuitry, amplifier circuitry, analog-to-digital converter circuitry,data output circuitry, memory (e.g., buffer circuitry), addresscircuitry, etc.

Still and video image data from camera sensor 14 may be provided toimage processing and data formatting circuitry 16 via path 28. Imageprocessing and data formatting circuitry 16 may be used to perform imageprocessing functions such as data formatting, adjusting white balanceand exposure, implementing video image stabilization, face detection,etc. Image processing and data formatting circuitry 16 may also be usedto compress raw camera image files if desired (e.g., to JointPhotographic Experts Group or JPEG format). In a typical arrangement,which is sometimes referred to as a system on chip (SOC) arrangement,camera sensor 14 and image processing and data formatting circuitry 16are implemented on a common semiconductor substrate (e.g., a commonsilicon image sensor integrated circuit die). If desired, camera sensor14 and image processing circuitry 16 may be formed on separatesemiconductor substrates. For example, camera sensor 14 and imageprocessing circuitry 16 may be formed on separate substrates that havebeen stacked.

Imaging system 10 (e.g., image processing and data formatting circuitry16) may convey acquired image data to host subsystem 20 over path 18.Host subsystem 20 may include processing software for detecting objectsin images, detecting motion of objects between image frames, determiningdistances to objects in images, filtering or otherwise processing imagesprovided by imaging system 10.

If desired, system 100 may provide a user with numerous high-levelfunctions. In a computer or cellular telephone, for example, a user maybe provided with the ability to run user applications. To implementthese functions, host subsystem 20 of system 100 may have input-outputdevices 22 such as keypads, input-output ports, joysticks, and displaysand storage and processing circuitry 24. Storage and processingcircuitry 24 may include volatile and nonvolatile memory (e.g.,random-access memory, flash memory, hard drives, solid-state drives,etc.). Storage and processing circuitry 24 may also includemicroprocessors, microcontrollers, digital signal processors,application specific integrated circuits, etc.

System 100 may be a vehicle safety system. In a vehicle safety system,images captured by the image sensor may be used by the vehicle safetysystem to determine environmental conditions surrounding the vehicle. Asexamples, vehicle safety systems may include systems such as a parkingassistance system, an automatic or semi-automatic cruise control system,an auto-braking system, a collision avoidance system, a lane keepingsystem (sometimes referred to as a lane drift avoidance system), apedestrian detection system, etc. In at least some instances, an imagesensor may form part of a semi-autonomous or autonomous self-drivingvehicle. Vehicle safety standards may require that the proper operationof any component of a vehicle safety system (including the image sensor)be verified before, during, and/or after operation of the vehicle.Verification operations for the image sensor may be performed by theimaging system prior to and/or after operation of a vehicle (e.g., uponstartup and/or shutdown of the imaging system). System 100 may also be asecurity system or any other desired type of system.

An example of an arrangement for camera module 12 of FIG. 1 is shown inFIG. 2. As shown in FIG. 2, camera module 12 includes image sensor 14and control and processing circuitry 44. Control and processingcircuitry 44 may correspond to image processing and data formattingcircuitry 16 in FIG. 1. Image sensor 14 may include a pixel array suchas array 32 of pixels 34 (sometimes referred to herein as image sensorpixels, imaging pixels, or image pixels 34). Control and processingcircuitry 44 may be coupled to row control circuitry 40 and may becoupled to column control and readout circuitry 42 via data path 26. Rowcontrol circuitry 40 may receive row addresses from control andprocessing circuitry 44 and may supply corresponding row control signalsto image pixels 34 over control paths 36 (e.g., dual conversion gaincontrol signals, pixel reset control signals, charge transfer controlsignals, blooming control signals, row select control signals, or anyother desired pixel control signals). Column control and readoutcircuitry 42 may be coupled to the columns of pixel array 32 via one ormore conductive lines such as column lines 38. Column lines 38 may becoupled to each column of image pixels 34 in image pixel array 32 (e.g.,each column of pixels may be coupled to a corresponding column line 38).Column lines 38 may be used for reading out image signals from imagepixels 34 and for supplying bias signals (e.g., bias currents or biasvoltages) to image pixels 34. During image pixel readout operations, apixel row in image pixel array 32 may be selected using row controlcircuitry 40 and image data associated with image pixels 34 of thatpixel row may be read out by column control and readout circuitry 42 oncolumn lines 38.

Column control and readout circuitry 42 may include column circuitrysuch as column amplifiers for amplifying signals read out from array 32,sample and hold circuitry for sampling and storing signals read out fromarray 32, analog-to-digital converter circuits for converting read outanalog signals to corresponding digital signals, and column memory forstoring the read out signals and any other desired data. Column controland readout circuitry 42 may output digital pixel values to control andprocessing circuitry 44 over line 26.

Array 32 may have any number of rows and columns. In general, the sizeof array 32 and the number of rows and columns in array 32 will dependon the particular implementation of image sensor 14. While rows andcolumns are generally described herein as being horizontal and vertical,respectively, rows and columns may refer to any grid-like structure(e.g., features described herein as rows may be arranged vertically andfeatures described herein as columns may be arranged horizontally).

If desired, array 32 may be part of a stacked-die arrangement in whichpixels 34 of array 32 are split between two or more stacked substrates.In such an arrangement, each of the pixels 34 in the array 32 may besplit between the two dies at any desired node within the pixel. As anexample, a node such as the floating diffusion node may be formed acrosstwo dies. Pixel circuitry that includes the photodiode and the circuitrycoupled between the photodiode and the desired node (such as thefloating diffusion node, in the present example) may be formed on afirst die, and the remaining pixel circuitry may be formed on a seconddie. The desired node may be formed on (i.e., as a part of) a couplingstructure (such as a conductive pad, a micro-pad, a conductiveinterconnect structure, or a conductive via) that connects the two dies.Before the two dies are bonded, the coupling structure may have a firstportion on the first die and may have a second portion on the seconddie. The first die and the second die may be bonded to each other suchthat first portion of the coupling structure and the second portion ofthe coupling structure are bonded together and are electrically coupled.If desired, the first and second portions of the coupling structure maybe compression bonded to each other. However, this is merelyillustrative. If desired, the first and second portions of the couplingstructures formed on the respective first and second dies may be bondedtogether using any known metal-to-metal bonding technique, such assoldering or welding.

As mentioned above, the desired node in the pixel circuit that is splitacross the two dies may be a floating diffusion node. Alternatively, thedesired node in the pixel circuit that is split across the two dies maybe the node between a floating diffusion region and the gate of a sourcefollower transistor (i.e., the floating diffusion node may be formed onthe first die on which the photodiode is formed, while the couplingstructure may connect the floating diffusion node to the source followertransistor on the second die), the node between a floating diffusionregion and a source-drain node of a transfer transistor (i.e., thefloating diffusion node may be formed on the second die on which thephotodiode is not located), the node between a source-drain node of asource-follower transistor and a row select transistor, or any otherdesired node of the pixel circuit.

FIG. 3 is an illustrative cross-sectional view of pixels 34 in imagesensor 14. Two representative pixels 34 are shown in FIG. 3. Each pixelmay include a respective photosensitive region 46 formed in a substratesuch as silicon substrate 48. The photosensitive regions may be formedby p-type or n-type doped silicon, for example. As shown in FIG. 3, eachpixel may include a respective photodiode (PD). Microlenses 50 may beformed over photodiodes 46 and may be used to direct incident lighttowards the photodiodes. Photosensitive regions 46 may serve to absorbincident light focused by microlenses 50 and produce pixel signals thatcorrespond to the amount of incident light absorbed.

A color filter layer 52 that includes color filter elements such ascolor filter elements 54 (sometimes referred to as color filters,plasmonic color filters, plasmonic color filter elements, plasmonicoptical elements, surface plasmon optical filters, surface plasmonicresonance filters, etc.) may be interposed between microlenses 50 andsubstrate 48. Color filter layer 52 may be formed from a metal layerhaving a plurality of openings 56. The example in FIG. 3 of a singlemetal layer forming color filter layer 52 is merely illustrative. Ifdesired, the color filter layer may include two or more metal layerswith intervening dielectric layers. Openings 56 extend entirely throughmetal layer 52 (e.g., from an upper surface of the metal layer to abottom surface of the metal layer). The arrangement and size of openings56 in the metal layer may be selected to form plasmonic color filterelements 54. When exposed to incident light, surface plasmon resonancewill occur on the plasmonic color filter elements and generate surfaceplasmon polaritons (SPPs). Consequently, the plasmonic color filterelements will allow transmission of a specific wavelength of light(based on the arrangement and size of openings 56) through the filterelement. Openings 56 may be circular, square, non-square rectangular, orany other desired shape. Openings 56 may be elongated slits that areelongated along a longitudinal axis, as an additional example.

As shown in FIG. 3, openings 56 may have widths 58. A first plasmoniccolor filter element may have openings with a width 58-1 whereas asecond plasmonic color filter element may have openings with a width58-2 that is smaller than 58-1. The openings may be separated bydistance 60. The first plasmonic color filter element may have openingsseparated by a distance 60-1 whereas the second plasmonic color filterelement may have openings separated by a distance 60-2 that is greaterthan distance 60-1. Distances 58-1 and 60-1 may be selected such thatlight of a first wavelength (e.g., red light) is transmitted through thecolor filter element towards the photodiode. Distances 58-2 and 60-2 maybe selected such that light of a second wavelength that is differentthan the first wavelength (e.g., blue light) is transmitted through thecolor filter element towards the photodiode. In other words, openings 56determine the transmission profile of light through the color filterelements. The color filter elements may transmit a narrow range ofwavelengths.

The distances 58 that define the widths of the openings in the colorfilter element layer may be any desired distances. For example, eachopening width 58 may be between 10 nanometers and 300 nanometers,between 100 nanometers and 300 nanometers, between 10 nanometers and 5microns, between 10 nanometers and 3 microns, greater than 10nanometers, greater than 1 nanometer, greater than 100 nanometers,greater than 1 micron, greater than 5 microns, less than 5 microns, lessthan 10 microns, less than 3 microns, less than 1 micron, less than 300nanometers, less than 10 nanometers etc. Similarly, each distance 60between openings may be between 10 nanometers and 300 nanometers,between 100 nanometers and 300 nanometers, between 10 nanometers and 5microns, between 10 nanometers and 3 microns, greater than 10nanometers, greater than 1 nanometer, greater than 100 nanometers,greater than 1 micron, greater than 5 microns, less than 5 microns, lessthan 10 microns, less than 3 microns, less than 1 micron, less than 300nanometers, less than 10 nanometers etc. The openings may be have awidth that is sufficiently small to block light from passing through theopenings directly (and only allow surface plasmon polaritons to passthrough the filter). This example, however, is merely illustrative. Ifdesired, some of the openings may have a width that allows light to passdirectly through the opening.

Color filter layer 52 may be formed from a metal that exhibits surfaceplasmon resonance effects. For example, color filter element layer 52may be formed from a metal (e.g., a noble metal) such as gold, silver,platinum, aluminum, copper, or a combination of one or more of thesematerials. The metal that forms color filter element 52 may have anegative dielectric constant. Color filter layer 52 may have a height62. Height 62 may be between 10 nanometers and 300 nanometers, between100 nanometers and 300 nanometers, between 10 nanometers and 5 microns,between 10 nanometers and 3 microns, greater than 10 nanometers, greaterthan 1 nanometer, greater than 100 nanometers, greater than 1 micron,greater than 5 microns, less than 5 microns, less than 10 microns, lessthan 3 microns, less than 1 micron, less than 300 nanometers, less than10 nanometers etc.

FIG. 3 shows optional dielectric materials formed on either side ofcolor filter layer 52. As shown in FIG. 3, a first dielectric layer 64is interposed between the upper surface of color filter layer 52 and thelower surfaces of microlenses 50. A second dielectric layer 66 isinterposed between the lower surface of color filter layer 52 and theupper surface of substrate 48. Dielectric layers 64 and 66 may be formedfrom air or any other desired dielectric material (e.g., polymer,silicon dioxide, etc.). Similarly, openings 56 in metal layer 52 may befilled air or any other desired dielectric material. Dielectric layers64 and 66 may be omitted if desired. For example, microlenses 50 may beformed directly on the upper surface of metal layer 52. Similarly, thelower surface of metal layer 52 may be formed directly on substrate 48.

The image sensor shown in FIG. 3 may suffer from cross-talk betweenadjacent pixels. Surface plasmon polaritons may propagate across theupper surface of metal layer 52 and reach the color filter elements ofadjacent pixels. To prevent cross-talk without sacrificing fill factor,metal walls may be included in between adjacent plasmonic color filterelements.

FIGS. 4A-4C show top views of illustrative image sensors havingplasmonic color filter elements. FIGS. 5A-5C are corresponding graphsshowing power as a function of wavelength for the image sensors of FIGS.4A-4C. FIG. 4A shows an image sensor having plasmonic color filterelements of the type shown in FIG. 3. As shown in FIG. 4A, each pixelmay be covered by a respective color filter element. The color filterelements are arranged in a 2×2 unit square that is repeated across theimage sensor. Plasmonic color filter element 54-1 is formed in theupper-left of the unit square, plasmonic color filter element 54-2 isformed in the upper-right of the unit square, plasmonic color filterelement 54-3 is formed in the lower-left of the unit square, andplasmonic color filter element 54-4 is formed in the lower-right of theunit square. In the example of FIG. 4A, color filter element 54-1transmits green light, color filter element 54-2 transmits red light,color filter element 54-3 transmits blue light, and color filter element54-4 transmits green light. This arrangement is sometimes referred to asa Bayer color filter pattern. This example is merely illustrative, andany desired color filter pattern may be used.

FIG. 5A is a graph showing power as a function of wavelength for theimage sensor of FIG. 4A. Curve 72 shows the response of the pixel havingthe blue color filter element (e.g., color filter element 54-3 in FIG.4A) to incident light, curve 74 shows the response of the pixel havingthe green color filter element (e.g., color filter element 54-1 in FIG.4A) to incident light, and curve 76 shows the response of the pixelhaving the red color filter element (e.g., color filter element 54-2 inFIG. 4A) to incident light. As shown, due to cross-talk caused bypropagation of surface plasmon polaritons to adjacent pixels, there issignificant overlap between the curves. It is therefore difficult todistinguish between blue and green light and between green and red lightusing the pixels of FIG. 4A.

FIG. 4B shows an image sensor with reduced cross-talk compared to theimage sensor of FIG. 4A. To reduce cross-talk, the separation betweenthe color filter elements (and underlying photosensitive areas) isincreased (as shown in FIG. 4B). This is referred to as lowering thefill factor (e.g., the ratio of a pixel's light sensitive area to itstotal area) of the pixels. FIG. 5B is a graph showing power as afunction of wavelength for the image sensor of FIG. 4B. Curve 82 showsthe response of the pixel having the blue color filter element (e.g.,color filter element 54-3 in FIG. 4B) to incident light, curve 84 showsthe response of the pixel having the green color filter element (e.g.,color filter element 54-1 in FIG. 4B) to incident light, and curve 86shows the response of the pixel having the red color filter element(e.g., color filter element 54-2 in FIG. 4B) to incident light. Asshown, lowering the fill factor reduces the cross-talk caused bypropagation of surface plasmon polaritons to adjacent pixels. However,the dynamic range (and quantum efficiency) of the image sensor issacrificed in the sensor of FIG. 4B (because there is a lower responsefrom the pixels for the same amount of light compared to the imagesensor of FIG. 4A). As shown in FIG. 5B, the maximum powers of curves82, 84, and 86 are less than the maximum powers of curves 72, 74, and76.

FIG. 4C shows an image sensor with reduced cross-talk compared to theimage sensor of FIG. 4A without sacrificing fill factor as in the imagesensor of FIG. 4B. To reduce cross-talk, metal walls 102 (sometimesreferred to as metal structures 102, metal sidewalls 102, or reflectivewalls 102) are formed between each adjacent color filter element. Themetal walls may be considered a part of the underlying color filterelements or may be considered a separate structure from the underlyingcolor filter elements. The metal walls are formed in a grid across theentire image sensor, for example. In this embodiment, each pixel has ametal wall formed around the periphery of the pixel. FIG. 5C is a graphshowing power as a function of wavelength for the image sensor of FIG.4C. Curve 92 shows the response of the pixel having the blue colorfilter element (e.g., color filter element 54-3 in FIG. 4C) to incidentlight, curve 94 shows the response of the pixel having the green colorfilter element (e.g., color filter element 54-1 in FIG. 4C) to incidentlight, and curve 96 shows the response of the pixel having the red colorfilter element (e.g., color filter element 54-2 in FIG. 4C) to incidentlight. As shown, adding the metal walls reduces the cross-talk caused bypropagation of surface plasmon polaritons to adjacent pixels.Additionally, the dynamic range (and quantum efficiency) of the imagesensor is not sacrificed as in the sensor of FIG. 4B (because the fillfactor is maintained at a similar level as in the sensor of FIG. 4A). Asshown in FIG. 5C, the maximum powers of curves 92, 94, and 96 are equalto or greater than the maximum powers of curves 72, 74, and 76.

FIG. 6 is a cross-sectional side view of an image sensor with plasmoniccolor filter elements separated by metal walls (similar to as shown inFIG. 4C, for example). The image sensor in FIG. 6 has similar featuresas the image sensor in FIG. 3. For example, photosensitive regions 46are formed in substrate 48. Metal color filter layer 52 with openings 56that forms plasmonic color filter elements is formed over thephotosensitive regions. Dielectric layer 66 may optionally be interposedbetween the color filter layer and the substrate.

In FIG. 6, however, metal walls 102 are formed in between adjacentpixels 34. Metal walls 102 may be formed over the upper surface of colorfilter layer 52 with openings 56. In one possible embodiment, metalwalls 102 may be formed in the same processing step as metal layer 52and may be formed integrally with metal layer 52 (e.g., a single metallayer may be patterned to form openings 56 for the color filter elementsand metal walls 102 for cross-talk reduction). In another embodiment,metal walls 102 may be formed in a different processing step than metallayer 52 (e.g., a separate metal layer of the same or a differentmaterial than metal layer 52 may be formed on the upper surface of metallayer 52 to form metal walls 102). Metal walls 102 may be formed usingsemiconductor processing techniques. For example, a metal layer may bedeposited over layer 52 and etched (or lift off) to form the metalwalls.

Metal walls 102 may block surface plasmon polaritons from a given colorfilter element from passing to an adjacent color filter element (byreflecting the surface plasmon polaritons). Metal walls 102 may alsoenhance the resonance effect and increase transmission of plasmoniccolor filters 54 (by reflecting the surface plasmon polaritons andkeeping them within the color filter element). This may also reduce thenumber of periodic structures required (e.g., less openings 56 will berequired) to form the plasmonic color filter element.

Metal walls 102 may have any desired height (e.g., the distance betweenthe upper surface of the metal wall and the upper surface of metal layer52). The height of the metal walls may be between 10 nanometers and 300nanometers, between 100 nanometers and 300 nanometers, between 10nanometers and 5 microns, between 10 nanometers and 3 microns, greaterthan 10 nanometers, greater than 1 nanometer, greater than 100nanometers, greater than 1 micron, greater than 5 microns, less than 5microns, less than 10 microns, less than 3 microns, less than 1 micron,less than 300 nanometers, less than 10 nanometers etc. Metal walls 102may have any desired thickness. The thickness of the metal walls may bebetween 10 nanometers and 300 nanometers, between 100 nanometers and 300nanometers, between 10 nanometers and 5 microns, between 10 nanometersand 3 microns, greater than 10 nanometers, greater than 1 nanometer,greater than 100 nanometers, greater than 1 micron, greater than 5microns, less than 5 microns, less than 10 microns, less than 3 microns,less than 1 micron, less than 300 nanometers, less than 10 nanometersetc.

Dielectric material 64 may be formed between metal walls 102, as shownin FIG. 6. Dielectric material 64 may have a low refractive index.Dielectric material 64 may be formed from air. In FIG. 6, the uppersurfaces of metal walls 102 are in direct contact with the lowersurfaces of microlenses 50. This example is merely illustrative. Ifdesired, an additional dielectric layer may be interposed between theupper surfaces of metal walls 102 and the lower surfaces of microlenses50.

One of the advantages of the plasmonic color filter elements shown inFIG. 6 is the flexibility to form different types of color filterelements with minimal variation to manufacturing processes. Color filterelements for transmitting light of a wide variety of wavelengths (e.g.,between 200 nanometers and 2 microns) may be formed using a single metallayer 52. This helps mitigate manufacturing time and costs. Imagesensors with two or more color filter types may sometimes be referred toas multi-spectral image sensors. FIG. 7 is a top view of an illustrativemulti-spectral image sensor having a color filter pattern with a 3×3unit square that is repeated across the image sensor. As shown in FIG.7, color filter elements 54-1, 54-2, 54-3, 54-4, 54-5, 54-5, 54-6, 54-7,54-8, and 54-9 are arranged in a 3×3 square. Each color filter elementis surrounded by metal walls 102. Each color filter element transmits arespective wavelength of light. Color filter element 54-1 transmitslight at a first wavelength (λ₁), color filter element 54-2 transmitslight at a second wavelength (λ₂), color filter element 54-3 transmitslight at a third wavelength (λ₃), color filter element 54-4 transmitslight at a fourth wavelength (λ₄), color filter element 54-5 transmitslight at a fifth wavelength (λ₅), color filter element 54-6 transmitslight at a sixth wavelength (λ₆), color filter element 54-7 transmitslight at a seventh wavelength (λ₇), color filter element 54-8 transmitslight at a eighth wavelength (λ₈), and color filter element 54-9transmits light at a ninth wavelength (λ₉). The nine wavelengths of FIG.7 may all be different (e.g., due to different opening sizes andarrangements in the respective color filter elements).

The arrangement of FIG. 7 is merely illustrative. In general, an imagesensor may include plasmonic color filter elements. The plasmonic colorfilter elements may have walls around their peripheries to preventcross-talk. An image sensor with plasmonic color filter elements of thistype may include any desired number of different types of color filterelements. For example, the image sensor may be a monochrome image sensor(with plasmonic color filter elements that all transmit light of thesame wavelength). The image sensor may be a multi-spectral image sensorwith two or more types of plasmonic color filter elements that transmitlight of two or more respective wavelengths. For example, the imagesensor may include plasmonic color filter elements arranged in a Bayercolor filter pattern (with three types of color filters eachtransmitting a respective wavelength of light). The image sensor mayinclude four or more different types of plasmonic color filter elements,six or more different types of plasmonic color filter elements, eight ormore different types of plasmonic color filter elements, ten or moredifferent types of plasmonic color filter elements, etc.

In various embodiment, an image sensor may include an array of imagingpixels and each imaging pixel may include a photosensitive region, amicrolens formed over the photosensitive region, a plasmonic colorfilter element interposed between the microlens and the photosensitiveregion, and metal walls that extend around a periphery of the plasmoniccolor filter element.

The plasmonic color filter element may include a metal layer (or two ormore stacked metal layers having intervening dielectric layers) having aplurality of openings. The metal layer may include a material selectedfrom the group consisting of: gold, silver, platinum, aluminum, andcopper. Each opening of the plurality of openings may have a widthbetween 10 nanometers and 300 nanometers. The metal layer may have anupper surface and a lower surface and the metal walls may be formed overthe upper surface of the metal layer. Each opening of the plurality ofopenings may extend from the upper surface of the metal layer to thelower surface of the metal layer. The metal walls may extend upwardsfrom the upper surface of the metal layer towards the microlens. Theimage sensor may also include a material formed between the metal wallsover the upper surface of the metal layer. The material may include air.The image sensor may also include a dielectric layer interposed betweenthe plasmonic color filter element and the photosensitive region. Themetal walls may include a material selected from the group consistingof: gold, silver, platinum, aluminum, and copper.

In various embodiments, an image sensor may include an array of imagingpixels and each imaging pixel may include a photosensitive region, amicrolens formed over the photosensitive region, and a plasmonic colorfilter element interposed between the microlens and the photosensitiveregion. The plasmonic color filter element may have one or more metallayers (e.g., a single metal layer or stacked metal layers havingdielectric layers in between) with openings and metal walls thatsurround the metal layer. Each opening of the plurality of openings mayhave a width that is less than 2 microns.

In various embodiments, a color filter element may include a metal layerwith an upper surface, a lower surface, and a periphery, a plurality ofopenings in the metal layer (or stacked metal layers having dielectriclayers in between), and metal walls that extend above the upper surfaceof the metal layer and that run around the periphery of the metal layer.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention. Theforegoing embodiments may be implemented individually or in anycombination.

What is claimed is:
 1. An image sensor comprising an array of imagingpixels, wherein each imaging pixel comprises: a photosensitive region; amicrolens formed over the photosensitive region; a plasmonic colorfilter element interposed between the microlens and the photosensitiveregion; and metal walls that extend around a periphery of the plasmoniccolor filter element.
 2. The image sensor defined in claim 1, whereinthe plasmonic color filter element comprises one or more metal layershaving a plurality of openings.
 3. The image sensor defined in claim 2,wherein the one or more metal layers comprise a material selected fromthe group consisting of: gold, silver, platinum, aluminum, and copper.4. The image sensor defined in claim 2, wherein each opening of theplurality of openings has a width between 10 nanometers and 300nanometers.
 5. The image sensor defined in claim 1, further comprising:a dielectric layer interposed between the plasmonic color filter elementand the photosensitive region.
 6. The image sensor defined in claim 1,wherein the metal walls comprise a material selected from the groupconsisting of: gold, silver, platinum, aluminum, and copper.
 7. An imagesensor comprising an array of imaging pixels, wherein each imaging pixelcomprises: a photosensitive region; a microlens formed over thephotosensitive region; a plasmonic color filter element interposedbetween the microlens and the photosensitive region, wherein theplasmonic color filter element comprises one or more metal layers havinga plurality of openings, an upper surface, and a lower surface; andmetal walls that extend around a periphery of the plasmonic color filterelement, wherein the metal walls are formed over the upper surface ofthe one or more metal layers.
 8. The image sensor defined in claim 7,wherein each opening of the plurality of openings extends from the uppersurface of the one or more metal layers to the lower surface of the oneor more metal layers.
 9. The image sensor defined in claim 7, whereinthe metal walls extend upwards from the upper surface of the one or moremetal layers towards the microlens.
 10. The image sensor defined inclaim 7, further comprising: a material formed between the metal wallsover the upper surface of the one or more metal layers.
 11. The imagesensor defined in claim 10, wherein the material comprises air.
 12. Theimage sensor defined in claim 7, wherein the one or more metal layerscomprise a material selected from the group consisting of: gold, silver,platinum, aluminum, and copper.
 13. The image sensor defined in claim 7,wherein each opening of the plurality of openings has a width between 10nanometers and 300 nanometers.
 14. The image sensor defined in claim 7,further comprising: a dielectric layer interposed between the plasmoniccolor filter element and the photosensitive region.
 15. The image sensordefined in claim 7, wherein the metal walls comprise a material selectedfrom the group consisting of: gold, silver, platinum, aluminum, andcopper.
 16. An image sensor comprising an array of imaging pixels,wherein at least one of the imaging pixels comprises: a photosensitiveregion; a microlens formed over the photosensitive region; a plasmoniccolor filter element formed over the photosensitive region, wherein theplasmonic color filter element comprises one or more metal layers havingan upper surface and a lower surface; and metal walls that extend arounda periphery of the plasmonic color filter element, wherein the metalwalls are formed over the upper surface of the one or more metal layers.17. The image sensor defined in claim 16, wherein the one or more metallayers has a plurality of openings that extend from the upper surface tothe lower surface.
 18. The image sensor defined in claim 16, wherein theplasmonic color filter element is interposed between the microlens andthe photosensitive region.